Thin film producing method and light bulb having such thin film

ABSTRACT

When forming an optical thin film on a surface of a bulb of a light source such as an electric lamp or a discharge lamp, a thin film whose interface/surface is less rough is formed on a base having a spheroid shape. When forming a thin film on a base  2  with a spheroid shape, which is disposed in a vacuum chamber  4  of a film-forming device and spun on its rotation axis, an interface or a surface of the thin film is made less rough and the thickness distribution of the thin film is made smaller by setting a sputtering gas pressure to be in a range from 0.04 Pa to 5.0 Pa; by using, as a sputtering gas, a mixed gas of Ar gas and N 2  gas in which the N 2  gas is present at a partial pressure ratio of 1 to 6 assuming a partial pressure of the Ar gas is 100, or a mixed gas of Ar gas, N 2  gas, and O 2  gas in which the N 2  gas is present at a partial pressure ratio of 1 to 6 and O 2  gas is present at a partial pressure ratio of 6 or less assuming a partial pressure of the Ar gas is 100; by setting an input power applied at the start of thin film formation to be the greatest throughout a sputtering process; or by applying a negative bias to the base.

TECHNICAL FIELD

The present invention relates to a method for forming an optical thinfilm on a surface of a bulb of a light source such as an electric lampor a discharge lamp and to an electric lamp having this thin film.

BACKGROUND ART

As a method for enabling efficient use of the power consumed in anelectric lamp such as an incandescent lamp and a tungsten halogen lamp,there has been proposed providing an infrared reflection film on asurface of a bulb of the electric lamp so that infrared light, whichaccounts for 70% to 80% of the radiant energy, is selectively reflectedby the infrared reflection film and converged on a filament portion ofthe electric lamp to heat the filament coil while visible light emittedfrom the lamp is transmitted through the bulb (e.g., Journal ofIlluminating Engineering Society, pp.197-203, July 1980).

A practical-level electric lamp consuming 20% to 30% less power ascompared with an electric lamp providing the same intensity ofillumination (total luminous flux) has been realized by allowing thefilament coil of the electric lamp to be reheated by the infrared lightthat is selectively reflected by the infrared reflection film andeffectively converged on the filament coil after being emitted from thefilament coil owing to a suitable shape of the bulb.

A general method for forming such an infrared reflection film includessputtering, various evaporation methods, etc. For the purpose ofreducing the ratio of the infrared light escaping from the bulb as heatrays and/or transmitting only visible light selectively while reflectingas much infrared light as possible effectively, an optical-film-formingtechnique for forming, as a visible/infrared filter, an interferencemultilayer film having a laminated structure including at least onetransparent dielectric thin film having a high refractive index and atleast one transparent dielectric thin film having a low refractive indexhas been used. In this case, the difference between the actual thicknessand the desired thickness of the respective optical thin films, whichare formed on the bulb typically having a three-dimensional spheroidshape, needs to be sufficiently small, and also, the thicknessdistribution of the respective optical thin films needs to besufficiently small so that the resultant optical filter has a wavelengthselectivity.

In the interference multilayer film as described above, not only theprecise thickness of the respective optical thin films but alsomaterials used for the respective optical films are important forefficient infrared reflection. When the interference multilayer film isformed by sputtering under the general film-forming condition, i.e., byusing either Ar gas or a mixed gas of Ar gas and O₂ gas as a sputteringgas, there arises a problem that an interface or a surface of aresultant thin film becomes rough so that infrared light cannot bereflected sufficiently effectively. More specifically, infrared light isdiffused by the rough interface or surface of the interferencemultilayer film when reflected, thereby preventing the infrared lightfrom being converged on the firmament coil effectively, resulting in thereduced infrared-reflection efficiency. On the other hand, while a CVDmethod allows a thin film whose interface or surface is less rough to beobtained easily, it has many problems such that the absolute value ofthe thickness of a resultant thin film is not always controlledsufficiently; a base on which a thin film is to be formed needs to beheated; and, when forming a multilayer film including different types ofthin films, different gases and film-forming conditions need to be useddepending on the types of the thin films.

The preset invention aims to solve the above-mentioned problems. It isan object of the present invention to provide a method for forming athin film whose interface or surface is less rough on a base including aspheroid shape by sputtering and to provide an electric lamp providedwith this thin film.

DISCLOSURE OF INVENTION

In order to achieve the above-mentioned object, the first method forforming a thin film according to the present invention includesperforming sputtering while spinning the base on a spheroid axis,wherein a gas pressure is in a range from 0.04 Pa to 5.0 Pa.

The second method of the present invention is a method for forming athin film on a base including a spheroid shape, wherein a mixed gas ofAr gas and N₂ gas in which the N₂ gas is present at a partial pressureratio of 1 to 6 assuming a partial pressure of the Ar gas is 100, or amixed gas of Ar gas, N₂ gas, and O₂ gas in which the N₂ gas is presentat a partial pressure ratio of 1 to 6 and O₂ gas is present at a partialpressure ratio of 6 or less assuming a partial pressure of the Ar gas is100, is used as a sputtering gas while the base is being spun on aspheroid axis.

The third method of the present invention is a method for forming a thinfilm on a base including a spheroid shape, wherein an input powerapplied at a start of thin film formation is the greatest throughout asputtering process, the sputtering process being performed whilespinning the base on a spheroid axis.

The fourth method of the present invention is a method for forming athin film on a base including a spheroid shape, which includes applyinga negative bias to the base while spinning the base on a spheroid axis.

In the above-mentioned first to fourth methods, it is preferable thatthe base is a bulb of an electric lamp selected from tungsten halogenlamps and incandescent lamps.

Further, in the above-mentioned methods, it is preferable that thesputtering is radio frequency-sputtering.

Furthermore, in the above-mentioned methods, it is preferable that Ta₂O₅and SiO₂ are used as materials for forming a thin film.

Next, an electric lamp according to the present invention includes aninfrared reflection film having a surface roughness Ra ranging from 2.9nm to 20.0 nm.

Further, in the above-mentioned electric lamp, it is preferable that theinfrared reflection film is formed by sputtering.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating a method for forming a thin film accordingto one example of the present invention.

FIG. 2 is a front view showing a configuration of a bulb of an electriclamp according to one example of the present invention.

FIG. 3 is a cross-sectional view showing a configuration of a thin filmaccording to one example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

According to one example of the present invention, an interface or asurface of a thin film formed on a base including a spheroid shape canbe made less rough and the thickness distribution of the thin film alsocan be made smaller by setting a sputtering gas pressure to be in arange from 0.04 Pa to 5.0 Pa; by using, as a sputtering gas, a mixed gasof Ar gas and N₂ gas in which the N₂ gas is present at a partialpressure ratio of 1 to 6 assuming a partial pressure of the Ar gas is100, or a mixed gas of Ar gas, N₂ gas, and O₂ gas in which the N₂ gas ispresent at a partial pressure ratio of 1 to 6 and O₂ gas is present at apartial pressure ratio of 6 or less assuming a partial pressure of theAr gas is 100; by setting an input power applied at the start of thinfilm formation to be the greatest throughout a sputtering process; or byapplying a negative bias to the base.

Hereinafter, the present invention will be described in detail by way ofspecific examples. FIG. 1 shows a radio frequency-sputtering apparatusused in the following respective embodiments. This film-formingapparatus is provided with a radio frequency power supply 9. Inside avacuum chamber 4 of this apparatus, a base 2 having a spheroid shapeopposes a sputtering target 1 (film-forming source) of 8 inch×36 inchwith a shutter 5 intervening therebetween. The apparatus is furtherprovided with an inlet 6 for supplying Ar gas to the vacuum chamber 4,an inlet 7 for supplying N₂ gas chamber 4. Further, a direct-currentpower supply 3 is provided for applying a bias voltage to the base 2.During the film formation, the base 2 is spinning on the spheroid axisY.

In the embodiments described below, a ratio of a gas mentioned isexpressed as a ratio by volume.

(Embodiment 1)

In Embodiment 1, Ta₂O₅ thin films and SiO₂ thin films were formed on asurface of a bulb of an electric lamp so as to be laminated alternatelyby radio frequency-sputtering. In the present embodiment, the bulb wasrotated on the spheroid axis Y at a constant angular velocity so as toprevent nonuniform thickness distribution of the respective thin filmson the surface of the bulb. A mixed gas of Ar gas and O₂ gas was used asa sputtering gas when forming the Ta₂O₅ thin films while only Ar gas wasused as a sputtering gas when forming SiO₂ thin films. The applied inputpower was 0 W at the start of thin film formation and was graduallyincreased so as to reach its maximum of 4000 W after 1 minute. Thesputtering was performed at sputtering gas pressures of 0.03 Pa, 0.04Pa, 0.15 Pa, 0.4 Pa, 1.0 Pa, 2.0 Pa, 3.0 Pa, 4.0 Pa, 5.0 Pa, and 6.0 Pato form multilayer films (samples No.1 to No.9) each including eightthin films and having a thickness of about 1000 nm. The surfaceroughness Ra of the respective multilayer films was measured using anatomic force microscope (hereinafter, referred to as “AFM”). The resultsare shown in Table 1.

TABLE 1 Change in surface roughness Ra with sputtering gas pressuresputtering gas surface roughness Ra (Pa) (nm) 0.03 film could not beformed (no discharge occurred) multilayer film 1 0.04 7.7 multilayerfilm 2 0.15 6.3 multilayer film 3 0.4 4.8 multilayer film 4 1.0 7.9multilayer film 5 2.0 13.8 multilayer film 6 3.0 15.5 multilayer film 74.0 16.6 multilayer film 8 5.0 19.8 multilayer film 9 6.0 22.1 (surfaceroughness of the base: 2.0 nm)

As can be seen from the results shown in Table 1, the multilayer film 3formed at a sputtering gas pressure of 0.4 Pa has the smallest surfaceroughness Ra. When the surface roughness Ra is greater than 20.0 nm,infrared light is scattered considerably when it is reflected, resultingin a degraded infrared-reflection efficiency of the multilayer film asan infrared reflection film. Therefore, in the present embodiment, amultilayer film having a surface roughness of 20.0 nm or less isregarded as an effective multilayer film. Thus, it was confirmed thatthe multilayer films formed at a sputtering gas pressure of 0.04 to 5.0Pa are effective multilayer films. The sputtering gas pressure ispreferably in a range from 0.04 to 2.0 Pa and more preferably in a rangefrom 0.04 to 1.0 Pa.

(Embodiment 2)

In Embodiment 2, Ta₂O₅ thin films and SiO₂ thin films were formed on asurface of a bulb of an electric lamp so as to be laminated alternatelyby radio frequency-sputtering as in Embodiment 1. Embodiment 2 differsfrom Embodiment 1 in that the sputtering was performed at a sputteringgas pressure of 0.4 Pa, a mixed gas of Ar gas, N₂ gas, and O₂ gas wasused as a sputtering gas when forming the Ta₂O₅ thin films, and a mixedgas of Ar gas and N₂ gas was used as a sputtering gas when forming SiO₂thin films. Multilayer films 10 to 13 each including eight thin filmsand having a thickness of about 1000 nm were formed under theabove-mentioned conditions and the surface roughness Ra of therespective multilayer films was measured using AFM. The results areshown in Table 2 along with the data regarding the multilayer film 3formed in Embodiment 1.

TABLE 2 Change in surface roughness Ra with type of sputtering gassurface materials of type of sputtering gas roughness thin film (ratio)Ra (nm) multilayer film 3 Ta₂O₅ Ar/O₂ (100/6) 4.8 SiO₂ Ar multilayerfilm 10 Ta₂O₅ Ar/N₂/O₂ (100/1/6) 4.3 SiO₂ Ar/N₂ (100/1) multilayer film11 Ta₂O₅ Ar/N₂/O₂ (100/3/6) 3.5 SiO₂ Ar/N₂ (100/3) multilayer film 12Ta₂O₅ Ar/N₂/O₂ (100/6/6) 4.0 SiO₂ Ar/N₂ (100/6) multilayer film 13 Ta₂O₅Ar/N₂/O₂ (100/10/6) 4.7 SiO₂ Ar/N₂ (100/10) (surface roughness of thebase: 2.0 nm)

As can be seen from the results shown in Table 2, the multilayer film 11formed using the mixed gas containing Ar gas, N₂ gas, and O₂ gas inwhich the Ar gas, N₂ gas, and O₂ gas are present at a ratio of 100:3:6when forming the Ta₂O₅ thin films, and using the mixed gas of Ar gas andN₂ gas in which the Ar gas and N₂ gas are present at a ratio of 100:3when forming the SiO₂ thin films, has the smallest surface roughness Ra.In the present embodiment, a multilayer film having a smaller surfaceroughness Ra than the multilayer film 3 formed using the sputteringgases containing no N₂ gas is regarded as an effective multilayer film.Thus, it was confirmed that the multilayer film formed using a mixed gasof Ar gas, N₂ gas, and O₂ gas in which 1 to 6 parts by volume of N₂ gasand 6 or less parts by volume of O₂ gas are present with respect to 100parts by volume of Ar gas when forming the Ta₂O₅ thin films, and using amixed gas of Ar gas and N₂ gas in which 1 to 6 parts by volume of N₂ gasare present with respect to 100 parts by volume of Ar gas when formingthe SiO₂ thin films, are effective multilayer films.

(Embodiment 3)

In Embodiment 3, Ta₂O₅ thin films and SiO₂ thin films were formed on asurface of a bulb of an electric lamp so as to be laminated alternatelyby radio frequency-sputtering as in Embodiment 2. Embodiment 3 differsfrom Embodiment 2 in that the input power applied at the start of thinfilm formation was 4000 W. A multilayer film 14 including eight thinfilms and having a thickness of about 1000 nm was formed under theabove-mentioned conditions and the surface roughness Ra of themultilayer film 14 was measured using AFM. The result is shown in Table3 along with the data regarding the multilayer film 11 formed inEmbodiment 2.

TABLE 3 Change in surface roughness Ra with input power at the start ofthin film formation input power at the start of surface roughness Rathin film formation (W) (nm) multilayer film 11   0 3.5 multilayer film14 4000 3.2 (surface roughness of the base: 2.0 nm)

As can be seen from the result shown in Table 3, the multilayer film 14formed under the condition in which the electric power applied at thestart of thin film formation was 4000 W has a smaller surface roughnessRa than the multilayer film formed under the condition in which theapplied electric power was gradually made greater from the start of thinfilm formation.

(Embodiment 4)

In Embodiment 4, Ta₂O₅ thin films and SiO₂ thin films were formed on asurface of a bulb of an electric lamp so as to be laminated alternatelyby radio frequency-sputtering as in Embodiment 3. Embodiment 4 differsfrom Embodiment 3 in that a bias voltage of −20 V was applied to thebulb. A multilayer film 15 including eight thin films and having athickness of about 1000 nm was formed under the above-mentionedconditions and the surface roughness Ra of the multilayer film 15 wasmeasured using AFM. The result is shown in Table 4 along with the dataregarding the multilayer film 14 formed in Embodiment 3.

TABLE 4 Change in surface roughness Ra with bias voltage applied to basebias voltage surface roughness Ra (V) (nm) multilayer film 14    0 3.2multilayer film 15 −20 V 2.9 (surface roughness of the base: 2.0 nm)

As can be seen from the result shown in Table 4, the multilayer film 15formed under the condition in which a bias voltage of −20 V was appliedto the bulb has a smaller surface roughness Ra than the multilayer film14 formed without applying a bias voltage to the bulb.

FIG. 2 is a plan view showing the bulb 10 of the electric lamp used inthe present embodiment. FIG. 3 is an enlarged cross-sectional viewshowing the multilayer film formed on the surface of the bulb (glassbulb) 10. In the present embodiment, SiO₂ thin films 12 and Ta₂O₅ thinfilms 13 are laminated alternately on the surface of the glass bulb 11.Either the SiO₂ thin film 12 or the Ta₂O₅ thin film 13 may be theoutermost layer of the multilayer film.

According to the above-mentioned method, it is possible to provide anelectric lamp having a thin film, i.e., an infrared reflection film,with a surface roughness Ra of 2.9 nm to 20.0 nm. When the surfaceroughness Ra is greater than 20.0 nm, infrared light is scatteredconsiderably to degrade the infrared-reflection efficiency of the thinfilm. On the other hand, when the surface roughness Ra is smaller than2.9 nm, the thin film is prone to be peeled off because adhesion betweenthe thin film and the surface of the glass bulb becomes weaker. Thesurface roughness Ra is preferably in a range from 2.9 nm to 15 nm andmore preferably in a range from 2.9 nm to 10 nm.

It is to be noted that the electric lamp is not limited to a specifictype and includes all types of electric lamps using a filament as anilluminant, such as krypton lamps and tungsten halogen lamps.

Industrial Applicability

As specifically described above, the method for forming a thin film on abase having a spheroid shape according to the present invention canprovide a thin film having a small surface roughness Ra. Therefore, themethod of the present invention can improve the reflection efficiency ofan infrared reflection film and thus can improve performance of theinfrared reflection film.

Further, by setting the surface roughness Ra of an infrared reflectionfilm to be in a range from 2.9 nm to 20.0 nm, an electric lamp of highperformance with improved reflection efficiency can be obtained.

1. A method for forming a thin film on a base including a spheroid shapecomprising: performing sputtering while spinning the base on a spheroidaxis; and applying a negative bias to the base while spinning the baseon a spheroid axis, wherein the sputtering is radiofrequency-sputtering, and wherein a gas pressure is in a range from 0.04Pa to 5.0 Pa.
 2. The method for forming a thin film according to claim1, wherein an input power applied at a start of thin film formation isthe greatest throughout a sputtering process, the sputtering processbeing performed while spinning the base on a spheroid axis.
 3. Themethod for forming a thin film according to claim 2, wherein the base isa bulb of an electric lamp selected from tungsten halogen lamp andincandescent lamps.
 4. The method for forming a thin film according toclaim 1, wherein the base is a bulb of an electric lamp selected fromtungsten halogen lamps and incandescent lamps.
 5. The method for forminga thin film according to claim 1, wherein Ta₂O₅ and SiO₂ are used asmaterials for forming a thin film.
 6. A method for forming a thin filmon a base including a spheroid shape, wherein a mixed gas of Ar gas andN₂ gas in which the N₂ gas is present at a partial pressure ratio of 1to 6 assuming a partial pressure of the Ar gas is 100, or a mixed gas ofAr gas, N₂ gas, and O₂ gas in which the N₂ gas is present at a partialpressure ratio of 1 to 6 and O₂ gas is present at a partial pressureratio of 6 or less assuming a partial pressure of the Ar gas is 100, isused as a sputtering gas while the base is being spun on a spheroidaxis, wherein the sputtering is radio frequency-sputtering.
 7. Themethod for forming a thin film according to claim 6, wherein the base isa bulb of an electric lamp selected from tungsten halogen lamps andincandescent lamps.
 8. The method for forming a thin film according toclaim 6, wherein Ta₂O₅ and SiO₂ are used as materials for forming a thinfilm.
 9. The method for forming a thin film according to claim 6,wherein a SiO₂ thin film is formed using SiO₂ as a material and, as asputtering gas, the mixed gas of Ar gas and N₂ gas is which the N₂ gasis present at the partial pressure ratio of 1 to 6 assuming the partialpressure of the Ar gas is 100, and a Ta₂O₅ thin film is formed usingTa₂O₅ as a material and, as a sputtering gas, the mixed gas of Ar gas,N₂ gas, and O₂ gas in which the N₂ gas is present at the partialpressure ratio of 1 to 6 and O₂ gas is present at the partial pressureratio of 6 or less assuming the partial pressure of the Ar gas is 100.10. The method for forming a thin film according to claim 6, wherein aninput power applied at a start of thin film formation is the greatestthroughout a sputtering process, the sputtering process being performedwhile spinning the base on a spheroid axis.
 11. The method for forming athin film according to claim 6, comprising: applying a negative bias tothe base while spinning the base on a spheroid axis.
 12. A method forforming a thin film on a base including a spheroid shape comprising:performing sputtering while spinning the base on a spheroid axis,wherein the sputtering is radio frequency-sputtering, wherein a gaspressure is in a range from 0.04 Pa to 5.0 Pa, and wherein Ta₂O₅ andSiO₂ are used as materials for forming a thin film.
 13. A method forforming a thin film on a base including a spheroid shape comprising:performing sputtering while spinning the base on a spheroid axis,wherein the sputtering is radio frequency-sputtering, wherein a gaspressure is in a range from 0.04 Pa to 5.0 Pa, wherein an input powerapplied at a start of thin film formation is the greatest throughout asputtering process, the sputtering process being performed whilespinning the base on a spheroid axis, and wherein Ta₂O₅ and SiO₂ areused as materials for forming a thin film.